1. Field of the Invention
The present invention relates to speed change gears adopting an internal-meshing planetary gear construction. The speed change gears are usually used as reduction gears, but they serve as speedup gears when their input and output are reversed.
2. Description of the Prior Art
Heretofore, reduction gears adopting an internal-meshing planetary gear construction as illustrated in FIG. 5 have been proposed in, for example, Japanese Patent Application Laid-open No. 272348/1991 or No. 244852/1991 or Japanese Utility Model Registration Application Laid-open No. 47454/1990.
In the reduction gears, all members are made of plastics, and the rotation of an input shaft 1 is derived as the reduced speed rotation of an output shaft 8 (a pinion gear 9).
The input shaft 1 is integrally formed with an eccentric element 2, round which an external gear 3 is rotatably supported.
The external gear 3 is integrally provided with external teeth 3a of trochoidal tooth profile at its outer periphery. Besides, a plurality of inner pins 3b in the shape of a circular cylinder are formed integrally with the external gear 3 so as to protrude on a circumference which is coaxial with the input shaft 1.
An internal gear 4 is integrally formed with teeth of circular-arc tooth profile 4a which internally mesh with the external teeth 3a of the external gear 3. The internal gear 4 serves also as a main casing, and is accordingly held in a fixed state in the exemplified reduction gears.
One end part of the output shaft 8 is integrally formed with a disc-shaped flange part 8d. The flange part 8d is formed with inner pin holes 8a in which the respective inner pins 3b are loosely fitted.
The output shaft 8 is supported by the supporting part 7a of a support cover 7, while the input shaft 1 is rotatably supported in a bearing hole 8c which is formed in the end part of the output shaft 8. In addition, the pinion gear 9 is assembled to the other end part 8b of the output shaft 8 remote from the flange part 8d so as to rotate along with the output shaft 8 and not to come away therefrom.
Next, the operation of the prior-art reduction gears will be briefly explained. When the input shaft 1 is rotated by one revolution, the eccentric element 2 also rotated by one revolution. Owing to the revolution of the eccentric element 2, the external gear 3 is about to rotate rockingly (swayingly) around the input shaft 1. Since, however, the external gear 3 has the rotation on its own axis restrained by the internal gear 4, it undergoes almost only rocking (swaying), internally meshing with the internal gear 4.
By way of example, assuming now the numbers of the teeth of the external gear 3 and the internal gear 4 to be N and (N+1), respectively, the difference between the numbers of the teeth is 1 (one). Consequently, the external gear 3 shifts (rotates on its own axis) to the amount of one tooth relative to the internal gear 4 (held in the fixed state) every revolution of the input shaft 1. This signifies that the rotation of the input shaft 1 is reduced to -1/N of the rotating speed of the external gear 3 (where the minus sign indicates the reverse rotation).
The rocking (swaying) component of the rotation of the external gear 3 is absorbed by clearances respectively defined between the inner pins 3b and the corresponding inner pin holes 8a, and only the component of the rotation of the same 3 on the axis thereof is transmitted to the output shaft 8 through the inner pins 3a. As a result, the speed reduction at the reduction gear ratio of -1/N is ultimately achieved.
The rotation of the output shaft 8 is transmitted to the pinion gear 9 which is fixed to the end part 8b of this output shaft 8. Since the gear 27 of a machine opposite to the reduction gears is held in mesh with the pinion gear 9, the output of these reduction gears is transmitted to the opposite (mating) machine.
The reduction gears of the internal-meshing planetary gear type as stated above, however, has the problem that the pinion gear 9 needs to be separately prepared for the purpose of permitting the power-wise connection of the reduction gears with the opposite machine. This problem is ascribable to the fact that, when the pinion gear 9 formed with external teeth is molded integrally with the output shaft 8 beforehand, the assembly of the reduction gears (especially, the assembly of the support cover 7) becomes impossible.
Another problem is that, in the case of separately assembling the pinion gear 9 to the output shaft 8 as stated above, the reduction gears need to be furnished with mechanisms (functions) which prevent the pinion gear 9 from rotating relative to the output shaft 8 and from coming away from the output shaft 8.
A further problem is that, since the reduction gears receive a reaction (momental load) from the side of the gear 27 of the opposite machine, they structurally need to prolong the output shaft supporting part 7a of the support cover 7 and to ensure a high rigidity for the supporting part 7a, so the overall length and the weight thereof increase.
A still further problem is that, since the output shaft has the flange part 8d and presents a complicated shape in three dimensions as apparent from the figure, the handling thereof during storage and transportation is inconvenient. The support cover 7 which presents such a complicated shape has the similar problem.